1. Field of the Invention
This invention relates to a method for improving the performance of an electrophoretic display and a microparticle forming process for the preparation of an electrophoretic dispersion. The dispersion may be used in all types of electrophoretic displays including transmissive, reflective and transflective displays. It may also be used in an electrophoretic display having the traditional up/down switching mode, the in-plane switching mode, or the dual switching mode, and in the total internal reflection (TIR) type of electrophoretic displays.
2. Description of Related Art
The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing charged pigment particles suspended in a colored dielectric solvent. This type of display was first proposed in 1969. An EPD typically comprises a pair of opposed, spaced-apart plate-like electrodes, with spacers predetermining a certain distance between the electrodes. At least one of the electrodes, typically on the viewing side, is transparent. For the passive type of EPDs, row and column electrodes on the top (the viewing side) and bottom plates respectively, are needed to drive the displays. In contrast, an array of thin film transistors (TFTs) on the bottom plate and a common, non-patterned transparent conductor plate on the top viewing substrate are required for the active type EPDs. An electrophoretic dispersion composed of a colored dielectric solvent with charged pigment particles dispersed therein is enclosed between the two plates.
When a voltage difference is imposed between the two plates, the pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus, the color showing at the transparent plate, determined by selectively charging the plates, can be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color. Intermediate color density (or shades of gray) due to intermediate pigment density at the transparent plate may be obtained by controlling the plate charge through a range of voltages.
EPDs of different pixel or cell structures have been reported previously, for example, the partition-type EPD (M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8):1148–1152 (1979)), the microencapsulated EPD (U.S. Pat. Nos. 5,961,804 and 5,930,026) and the total internal reflection (TIR) type of EPD using microprisms or microgrooves as disclosed in M. A. Mossman, et al, SID 01 Digest pp. 1054 (2001); SID IDRC proceedings, pp. 311 (2000); and SID′02 Digest, pp. 522 (2002).
An improved EPD technology was recently disclosed in U.S. application Ser. No. 09/518,488 (WO 01/67170), Ser. No. 09/606,654 (WO 02/01281), Ser. No. 09/784,972 (US Published Application No. 2002/0182544, WO 02/65215). The improved EPD comprises isolated cells formed from microcups of well-defined shape, size and aspect ratio and filled with charged pigment particles dispersed in a dielectric solvent, preferably a fluorinated solvent or solvent mixture. The filled cells are individually top-sealed with a polymeric sealing layer, preferably formed from a composition comprising a material selected from a group consisting of thermoplastics, thermosets and precursors thereof.
The microcup EPDs may have the traditional up/down switching mode, the in-plane switching mode or the dual switching mode. In the display having the traditional up/down switching mode or the dual switching mode, there are a top transparent electrode plate, a bottom electrode plate and a plurality of isolated cells enclosed between the two electrode plates. In the display having the in-plane switching mode, the cells are sandwiched between an insulator layer and an electrode plate.
For all types of the EPDs, the dispersion contained within the individual cells of the display is undoubtedly one of the most crucial parts of the device. The dispersion, as stated earlier, usually is composed of pigment particles dispersed in a dielectric solvent. The composition of the dispersion determines, to a large extent, the lifetime, contrast ratio, switching rate, response waveform and bistability of the device. In an ideal dispersion, the pigment particles remain separate and do not agglomerate or stick to the electrodes under all operating conditions. Furthermore, all components in the dispersion must be chemically stable and compatible not only with each other but also with the other materials present in an EPD, such as the electrodes and sealing materials.
The pigment particles in the dispersion may exhibit a native charge, or may acquire a charge when suspended in the dielectric solvent or may be charged using a charge controlling agent (CCA).
Halogenated solvents of high specific gravity have been widely used in EPD applications particularly in those involving an inorganic pigment, such as TiO2, as the charged white or colored particles. The halogenated solvents of high specific gravity are very useful in reducing the rate of sedimentation of the pigment particles in the solvent. Fluorinated solvents are among the most preferred because they are chemically stable and environmentally friendly. However, most CCAs and dispersants suitable for use in hydrocarbon solvents are not effective for dispersions in fluorinated solvents, particularly high boiling-point perfluorinated solvents. This could be due to poor solubility or charge separation of the CCAs in these solvents. As a result, pigment particles are very difficult to disperse in perfluorinated solvents. Therefore, EPDs based on perfluorinated dielectric solvents typically show poor stability and switching performance.
To improve the stability and display performance of EPDs based on fluorinated solvents, an electrophoretic dispersion comprising 22.5 to 44.25% by weight of a hydrocarbon solvent, 54.42–75.20% by weight of at least one chlorine-free fluorinated solvent and 0.1 to 1.5% by weight of a fluorinated surfactant, was disclosed in U.S. Pat. No. 5,573,711. The presence of a hydrocarbon solvent such as phenylxylylethane, phenyloctane, decahydronaphthalene or xylene was claimed to result in a stronger solvent system that gives a better display performance. However, the use of a hydrocarbon solvent in any significant amount is undesirable because it lowers the specific gravity of the solvent and, as a result, increases the sedimentation rate of the pigment particles particularly when a pigment of high specific gravity, such as TiO2, is used.
The whole content of each document referred to in this application is incorporated by reference into this application in its entirety.